The present invention relates to improvements in motion characteristics, fabrication method, and the hull arrangement of a floating system intended for operating at sea with wet or dry-tree risers. The hull improvement in the present invention is related to a pontoon arrangement which makes the fabrication process easier and faster.
The present invention is developed to answer challenges in shallow water and marginal fields. One of the key challenges in marginal field development is the uncertainty of future production, where, often, production declines faster than predicted. Because of that, a production facility should be able to be relocated to other fields such that the service life of the facility can be used according to its design life. In other words, economic valuation of the production facility would be much better if options for relocation are made available.
Another challenge in shallow water development is subsidence which may be caused by many reasons. One of which is shrinking of the reservoir after production over many years. Unfortunately, the preferred solution for production platforms in shallow water is a fixed platform (jacket) which is not designed to handle subsidence well. The present invention is expected to handle subsidence easily.
Earthquakes are another challenge for the fixed platform. In some parts of the world, the prevalence of earthquakes may require more complex and expensive fixed platform designs. The present invention, however, is not greatly influenced by earthquakes because its nature is a floating platform.
One of the existing solutions for shallow water and marginal field development is using conventional semi-submersible offshore platforms which comprise a hull that has sufficient buoyancy to support a work platform above the water surface, as well as rigid and/or flexible piping or risers. The hull typically comprises a plurality of horizontal pontoons that supports a plurality of vertically upstanding columns, which in turn support the work platform above the surface of the water. The horizontal pontoons are costly and complicated in terms of operation and fabrication.
In general, the conventional semi-submersible offshore platform incorporates a conventional catenary chain-link spread-mooring arrangement to maintain its position over the well site. The motions of these types of semi-submersible platforms are usually relatively large, and accordingly, they require the use of “catenary” risers (either flexible or rigid) extending from the seafloor to the work platform and the heavy wellhead equipment is typically installed on the sea-floor, rather than on the work platform. The risers have a catenary shape to absorb the large heave (vertical motions) and horizontal motions of the structure. Due to their large motions, conventional semi-submersible platforms usually do not support top-tensioned risers.
During drilling or production operations, it is generally desirable to minimize the motion of the offshore platform to maintain the position of the platform over the well site and to reduce the likelihood of damage to the risers. One component of offshore platform motion is heave, which is the vertical linear displacement of the offshore platform in response to wave motion. For use in conjunction with top tensioned risers or dry tree solutions, the floating structure preferably has heave characteristics such that the strokes (relative motion between the hull and the risers) and the tension of the risers are within acceptable limits. Further, for use in conjunction with steel catenary risers or wet tree solutions, the floating structure preferably has heave characteristics such that the riser fatigue and strength requirements are within acceptable limits.
Accordingly, there remains a need in the art for a semi-submersible offshore platform with acceptable heave characteristics, and which can be manufactured more cost effectively and can be operated efficiently.